Recently, various bases are printed. However, not all bases have excellent print suitability and further beautiful and high definition printing is required. At the same time, the base for a package is required to have heat resistance, retort resistance, water resistance, gas barrier properties, etc. in order to protect the contents therein.
As a method for improving the print suitability, there is a method (1) in which adhesion and transfer properties of the printing ink are improved by improving the surface of the base by a corona discharge treatment, a frame treatment, or a plasma treatment, and a method (2) in which the print suitability and adhesive properties are improved by forming a resin layer for providing high adhesive properties to the surface of the base. In order to improve the printing properties of a plastic base, the method (1) for improving the surface conditions by the corona discharge treatment, the frame treatment, or the plasma treatment is widely used, due to ease of the method (1), and popularization of the device used.
On the other hand, in the method (2), a urethane-based resin (for example, Patent Documents Nos. 1 and 2), a polyester-based resin (for example, Patent Documents Nos. 3 and 4), or an acryl polyol-based resin (for example, Patent Document No. 5), etc. is used to form a resin layer for applying the high adhesive properties. At the same time, an epoxy-based curing agent or an isocyanate-based curing agent is often used.
However, when the corona discharge treatment, or the frame treatment is used, the effects for improving the surface are decreased over time. In addition, the adhesive properties are insufficient under high humidities. Specifically, when an OPP film is subjected to the corona discharge treatment, or an ordinary frame treatment, the wettability at the surface of the film is about 50 mN/m just after the treatment. However, after six months, the wettability decreases to about 40 mN/m. Due to this, adhesive properties of the printing ink and dot reproducibility are often decreased.
In addition, as a special frame treatment, there is a method using silicic acid flame. In this method, the wettability at the surface of the base is improved to 70 mN/m or more by bonding an aminosilane with the surface of the base. The method can achieve beautiful and high definition printing. However, the treated surface has inferior rub resistance, hot water resistance, and adhesive properties under highly humid conditions. In addition, a printing ink and an adhesive used are required to have a hydrophilic group.
In the method using a urethane-based resin layer as a resin layer for providing high adhesive properties, adhesive properties are insufficient under highly humid conditions. As the method using a polyester-based resin layer as a resin layer for providing high adhesive properties, a method in which a high crystalline polyester and a low crystalline polyester are mixed or a high melting point-polyester and a low melting point-polyester are mixed is normally used. However, the former has a problem in that the adhesive properties are unstable, and resistance to hot water is insufficient.
When a urethane resin is formed on a PET film as a resin layer for providing high adhesive properties, adhesion of a urethane-based gravure ink for packages is remarkably improved. However, the adhesion of other resins, such as an acryl-based resin and the dot reproducibility are inferior.
A method in which a resin layer for providing high adhesive properties is made by combination between acryl polyol and an isocyanate-based curing agent improves adhesive properties between the base and a deposition layer by using an acryl resin (Patent Document No. 5). However, the resin layer for providing high adhesive properties may be dissolved by the printing ink containing an organic solvent, because the resin layer has insufficient solvent resistance. Due to this, printed images may be deteriorated. When the resin layer for providing high adhesive properties containing acryl polyol and an isocyanate-based curing agent is formed on a base having a high polarity, or the base having a high polarity is formed on the resin, and the laminate obtained is stored under highly humid conditions for a long period of time or immersed in water, the adhesive properties between the resin layer for providing high adhesive properties and the base may be decreased. In addition, when the adhesive interface in the laminate is wet with water in a short time, adhesive properties may be deteriorated. In particular, the adhesion is remarkably decreased under acidic conditions having pH 5 or less.